Various ring-shaped communication networks (circuit networks) have been proposed and have prevailed. For example, the LAN (Local Area Network) specification includes a so-called token ring (see ISO/IEC 8802-5: 1998).
Data transfer in a token ring system will be briefly described below. A node that wants to transmit data acquires a token called a free token, which is not occupied by any node, and is going around a ring bus. The node that acquired the free token copies a destination identifier and processing data to the token, and outputs that token onto the ring bus to transmit it to a destination module. As described above, in the token ring system, a frame called a token is transferred in turn from a node to another node, and then to the next node, which are allocated on the ring bus, thus transmitting that frame to a target terminal.
On the other hand, when a receiving node receives a token which has a destination as the self node, it copies processing data and enters the received token, which is set with a reception completion flag indicating completion of reception, again onto the ring bus. This entry is done to inform the transmission source node of successful data transfer, and the transmission source node re-transmits the identical token if the token which is not set with the reception completion flag is returned. In this way, the receiving node sets the reception completion flag in the token, and returns that token to the transmission source node, thus implementing data transfer.